The fire detection systems presently use many mines throughout the world are based only on the detection of carbon monoxide ("CO"). See "Proceedings of the 22nd International Conference of Safety in Mines Research Institutes," and particularly the article by W. Heyn and K. Holke entitled "Testing of Fire Detection Systems for the Early Detection of Fires in the Underground Belt Conveyor Road", Nov., 1987, at pp 515-520. Studies conducted by the U.S. Bureau of Mines show that CO is generated by fires and in mines can be used to detect them in the early stages of development since very small quantities of CO can be measured with present day sensors. For example, the studies show that the CO emanating from a smoldering fire rises over a period of about 8 minutes from 0 to 35 ppm. Thereafter, flames appear and the CO generated from the fire rises very rapidly.
The CO based fire detection systems typically send an alarm signal at a threshold of 10 to 15 ppm above the minimal ambient background levels. Although these detection systems, in many cases, are quite reliable, they have not been used with much success in mines where Diesel powered equipment is also operated. The problem is that the engine's exhaust of Diesel powered equipment contains CO which is misinterpreted by the detection system as a fire resulting in many false alarms. These false alarms severely reduce the utility of these CO based fire detection systems in mines using Diesel powered equipment since the alarm threshold level must be set so high to reduce the number of false alarms that the sensitiVity and early warning capability provided by the system is greatly reduced.
Other fire detection systems also take into account other gases which may be present in a mine such as H.sub.2, CO.sub.2, NO, CH.sub.4 and other hydrocarbons. See Eicker, "Early Fire Detection Using Trace Gases in the Airstream", Gluckauf and Translation 124 (1988) Nr. 10 at pp. 304-309. The problem with this approach is that it either requires the monitoring of a large number of gases or the use of expensive and complicated equipment such as a gas chromatograph.
Recently, the U.S. Bureau of Mines contemplated the development of a fire detection system that used the ratio of CO to oxides of nitrogen (NO.sub.x). See U.S. Bureau of Mines Information (Internal Report) entitled "The Merits of CO and CO/CO.sub.2 Ratios for Fire Detection In Diesel Operated Mines" by C. D. Litton. This proposed system is based on the fact that a Diesel engine produces both CO and NO, whereas a smoldering fire produces only CO. It also assumes that all Diesel engines produce CO and NO in a predictable and fixed ratio. The disadvantage with this proposed approach is that it assumes that all Diesel engines produce CO and NO at a predictable and fixed rate. Experimental results have determined that the CO and NO.sub.x levels in a mine are not predictable and do not maintain a fixed ratio. Rather, they are dynamic and change over time, varying from mine to mine and even from point to point within a given mine.
It would be desirable, therefore, to have a method and system for detecting fires in an underground mine using a CO based detection system which did not have the above-described disadvantages.